The present invention relates to a digital modulation signal reading device, and, particularly to a reading device for use in the reproduction of digital information whose PCM (pulse code modulation) signal is recorded on a recording medium or transmitted into a transmission medium according to a modulation system in which the time ratio of the high level signal duration to the low level signal duration within a unit time is substantially unity.
In recording a digital information signal such as a PCM signal on a recording medium, predetermined modulation is performed. In examples of modulation systems usable for this purpose, the time ratio of the high level signal duration to the low level signal duration within a unit time is substantially unity. According to this modulation system, there is an advantage in that the reproduced output has no direct current component. With the reproduction of such signals, a reproduced RF (radio frequency) signal from the pickup takes the form of a distorted sinusoidal wave due to the finite frequency characteristics of the pick-up or the recording medium. In order to wave-shape such a distorted waveform into a rectangular waveform, the so-called zero level comparation system including a level comparator has been used.
In an optical recording and reproducing system using an optical recording disc in which the existence or absence of a pit are made correspondent to low and high levels of a digital modulation signal, respectively, and such high and low levels are optically read out, the binary level is detected by detecting the amount of irradiating light reflected from the disc, and therefore the peak values of the high and low levels of a reproduced signal cannot be symmetrical (See FIG. 2b). Even if the reproduced level is compared with a constant comparison level (zero level) in a level comparator, it is impossible to obtain the exact original digital information.
In order to resolve this problem, it has been usual, as shown in FIG. 1, to detect the current component contained in the output of the level comparator 2, which acts to wave-shape an output of the reproduction amplifier 1, by using an integrator 3, to obtain an output corresponding to the direct current component from a differential amplifier 4. The latter output is used as the comparison level in the level comparator 2. That is, when the d.c. component contained in the output of the level comparator 2 increases in a positive direction, the time duration of the high level to that of the low level of the comparison output can be made equal by correspondingly increasing the comparison level of the comparator 2, since the high level time duration of the reproduction input is longer than the other. On the other hand, when the d.c. component in the comparison output increases in the negative direction, it is compensated by lowering the comparison level of the comparator 2 correspondingly. A circuit performing such an operation is referred to as an ATC (automatic threshold control) circuit and is disclosed in detail in (Japanese) patent application No. 56-215207 filed on Dec. 25, 1981 applied for by the present applicant.
FIG. 2a shows a recording signal waveform to be supplied to a recording medium according to the EFM (eight to fourteen modulation) system in which the time ratio of the high level to the low level is made substantially equal to unity. The FIG. 2b waveform is the reproduced waveform when the recording signal indicated by the FIG. 2a waveform is picked up and reproduced. The FIG. 2b waveform is a distorted sinusoidal wave whose positive and negative peaks are symmetrical to each other with respect to a zero level 5. The FIG. 2c waveform is a comparison output obtained by level comparison of the FIG. 2b waveform with the zero level 5, and is still different from the original signal of FIG. 2a, i.e., it is not an exact reproduction output. By supplying the FIG. 2b waveform to the ATC circuit in FIG. 1, the comparison level of the comparator 2 is automatically controlled as shown by a chain line 6, resulting in the same waveform as the waveform a being reproduced.
In this circuit system, the time constant of the integrator 3 functioning to detect the time ratio is necessarily large because this system utilizes the fact that the time ratio of the high level to the low level of the modulation signal is substantially unity over a long time basis. Therefore, the comparison level (threshold level) of the level comparison 2 cannot trace a level reduction phenomenon due to instantaneous drop out, etc. as shown by the straight line 6 in FIG. 5A. Consequently, a non-data section may be produced.
(Japanese) Patent Application Laid-Open No. 55-150644 laid-open for public inspection on Nov. 22, 1980 (no full examination has been requested) discloses a circuit system whose trace characteristic is improved by making the time constant of the ATC circuit smaller. FIG. 3 shows the circuit system schematically. The signal from the reproduction amplifier 1 is compared in the level comparator 2 with a predetermined comparison level as in the case of the circuit in FIG. 1. As the comparison level, an output of a synthesizer 9 is used which is obtained by, for example, summing therein outputs of peak hold circuits 7 and 8 which hold the positive and negative peak values of the reproduction signal, respectively, in an appropriate ratio.
In the case of a reproduction signal such as shown by curve 10 in FIG. 4, the positive and negative peak hold outputs become as shown by the chained curves 11 and 12, respectively, and an intermediate level between the positive and negative peaks, which is obtained by the adder 9, becomes as shown by curve 13.
In this circuit system, the time constants of the peak hold circuits 7 and 8 are very small compared with that of the circuit of FIG. 1, and thus the response time of the threshold level is shorter. Further, in the case of drop-out, the threshold level 14 changes as shown in FIG. 5B, causing no non-data section. When this system is used to wave-shape the reproduction signal from a recording medium having the asymmetry shown in FIG. 2b, however, there may be defects in normal operation, which will be described below.
That is, when the time constants of the peak hold circuits are made small enough to be capable of tracing the respective peaks of the signal, there is no level variation due to drop out, etc. when the amplitude of the reproduced RF signal 10 varies as shown in FIG. 4. However, the threshold level always varies as shown by curve 13 and is astable. Further since the threshold level 13 intersects with the RF signal 10 at points deviated from the real threshold level 15, it is impossible to obtain an exact reproduction waveform. On the other hand, when the time constants of the respective positive and negative peak hold circuits are made large enough to be capable of tracing in the drop out case while not capable of tracing the RF signal peaks, the threshold level becomes a line passing through the intermediate values of the respective peaks. A solid line 16 in FIG. 4 shows the latter case, which differs from the true threshold level 5, resulting in that exact reproduction is still impossible.